In a conventional polishing machine shown in FIG. 13, a wafer “W” is held on a holding face (a lower face) of a holding plate 100 of a rotatable top ring. The holding face is covered with a water-absorptive bucking member 106, e.g., a nonwoven fabric. A lower surface of the wafer “W” is pressed onto polishing cloth 104 adhered on a polishing plate 102. The top ring and the polishing plate 102 are rotated so as to polish the lower surface of the wafer “W”. On the holding face of the holding plate 100, a template 108 is provided along an outer edge of the holding plate 100. The template 100 holds the wafer “W” at the right position on the holding face while polishing the wafer “W”.
Slurry is supplied onto a polishing face (an upper face) of the polishing cloth 104, and the wafer “W” held by the holding plate 100 is pressed onto the polishing face with proper pressing force. In this state, the lower surface of the wafer “W” is polished by rotating the polishing plate 102.
However, as shown in FIG. 13, a depression 104a, which corresponds to the wafer “W”, is formed in the polishing cloth 104 by the pressing force. The lower outer edge of the wafer “W” is abraded by an inner corner 104b of the depression 104a. By abrading the edge of the wafer “W”, polishing accuracy of the edge of the wafer “W” must be low.
To reduce the bad influence caused by the depression 104a, an improved polishing machine was disclosed in U.S. Pat. No. 5,584,751. The improved machine will be explained with reference to FIG. 12.
In this improved polishing machine, a head section 200 comprises: a main body section 204 connected to a rotary shaft 201, which is vertically moved by elevating means (not shown), e.g., a cylinder unit, and rotated by rotating means (not shown), e.g., a motor; and a holding plate 210, which is provided in a concave part 206 of the main body part 204. An opening of the concave part 206 faces polishing cloth 205 adhered on a polishing plate (not shown). The holding plate 210 is suspended by an elastic sheet 208. Compressed air is supplied to and discharged from a space 211 formed between the elastic sheet 208 and inner faces of the concave part 206 by a compressor 215 via a pipe 214. With this structure, the holding plate 210 is vertically moved by adjusting air pressure in the space 211.
A retainer ring 212 is provided to a lower end of the main body part 204. The retainer ring 212 encloses the holding plate 210. The retainer ring 212 is suspended and connected to the main body part 204 by a donut-shaped elastic sheet 216. Compressed air is supplied to and discharged from a space 218 formed on the upper side of the elastic sheet 216 by a compressor 220 via a pipe 222. With this structure, the retainer ring 212 is vertically moved by adjusting air pressure in the space 218. An inner circumferential face of the retainer ring 212 slides on an outer circumferential face of the holding plate 210 while the retainer ring 212 is vertically moved. The vertical motion of the retainer ring 212 can be independently executed with respect to the holding plate 210.
A holding face of the holding plate 210 is covered with a water-absorptive bucking member 106, e.g., a nonwoven fabric. An inner circumferential face of the retainer ring 212 holds the wafer “W” at the right position on the holding face of the holding plate 210 while polishing the wafer “W”.
In the polishing machine shown in FIG. 12, the head section 200 is downwardly moved to a prescribed position by the elevating means so as to move the wafer “W”, which has been held on the bucking member 106 of the holding plate 210, close to the polishing cloth 205 of the polishing plate.
Then, the compressed air is supplied into the space 211 from the compressor 215 via the pipe 214 so as to downwardly move the holding plate 210 against the elasticity of the elastic sheet 208. With this action, the lower surface of the wafer “W” can be pressed onto the polishing cloth 205 with proper pressing force.
At that time, the compressed air is supplied into the space 218 by the compressor 220 via the pipe 222 so as to downwardly move the retainer ring 212 against the elasticity of the elastic sheet 216. With this action, the retainer ring 212 can be pressed onto the polishing cloth 205 with proper pressing force (load). The retainer ring 212 can be independently pressed with respect to the holding plate 210.
The head section 200 is rotated by the rotating means so as to polish the lower surface of the wafer “W” upon the application of the proper pressing force (load).
When the wafer “W” is polished, the pressing force (load) applied to the wafer “W” is different from that applied to the retainer ring 212. By pressing the retainer ring 212 enclosing the holding plate 210, the level of the polishing cloth 205 along the outer edge of holding plate 210, which is pressed by the retainer ring 212, can be made substantially equal to that of the polishing cloth 205 pressed by the wafer “W” as shown in FIG. 14. Therefore, the outer edge of the wafer “W” is not abraded by the inner corner 104b of the depression 104a (see FIG. 13), so that the polishing accuracy of the edge of the wafer “W” can be high.
Since the retainer ring 212 vertically slides on the outer circumferential face of the holding plate 210, the retainer ring 212 holds the wafer “W” at the right position on the holding face of the holding plate 210 while polishing the wafer “W”. Therefore, no template 108 (see FIG. 13) provided along the outer edge of the holding plate 210 is required.
However, in the head section 200 shown in FIG. 12, the holding plate 210 and the retainer ring 212 are suspended, in the main body section 204, by the elastic sheets 208 and 216.
Therefore, the holding plate 210 is rotated together with the retainer ring 212, so positional relationship between the holding plate 210 and the retainer ring 212 are maintained during rotation.
With this structure, if any damage exists in a bottom face of the retainer ring 212, which presses the polishing cloth 205, a surface condition of the polishing cloth 205, which is badly influenced by the damage, badly influences flatness of the polished wafer “W”.
Further, forming very fine projections in the bottom face of the retainer ring 212 is unavoidable due to machining accuracy, so the machining accuracy of the bottom face of the retainer ring 212 directly influences the polishing accuracy of the wafer “W”.
If the holding plate 210 and the retainer ring 212 are independently rotated at different speeds, the detrimental influence caused by the surface condition of the bottom face of the retainer ring 212 can be very small.
However, the structure of the head section 200 for independently rotating the holding plate 210 and the retainer ring 212 at different speeds must be complex. Further, two motors for independently rotating are required, so that the whole structure of the polishing machine must be complex.